In a hybrid vehicle (HEV), an electric vehicle (EV), or the like, an assembled battery (battery system) including serial connection of a plurality of single battery cells, each of which is a secondary battery, is generally used to secure an intended high-voltage. Conventionally, to such an assembled battery, a battery monitoring circuit including an integrated circuit or the like is connected to each predetermined number of single battery cells. By performing measurement of an interterminal voltage of each single battery cell (cell voltage) or performing a balancing discharge for equalization of remaining capacity of each single battery cell with the battery monitoring circuit, a state of each single battery cell is monitored and managed. During balancing, each single battery cell discharges current according to remaining capacity and a discharge current flows in a balancing resistor through a voltage detection line provided between each single battery cell and the battery monitoring circuit. Here, a voltage drop corresponding to impedance is generated in the voltage detection line.
Recently, a single battery cell with a smaller voltage variation with respect to a variation of remaining capacity is in practical use. When such a single battery cell is used, higher measurement accuracy is required in order to measure a cell voltage and to estimate remaining capacity accurately. Thus, in measurement of a cell voltage during balancing, an influence of a voltage drop in such a voltage detection line described above is getting more significant. Thus, a method of measuring a cell voltage accurately by correcting an amount of a voltage drop in a voltage detection line has been proposed (see PTL 1).